(i) Field of the Invention
The present invention relates to a method for kneading a rubber composition obtained by blending a rubber component with silica and a silane coupling agent.
(ii) Description of the Prior Art
Carbon black has so far been used as a reinforcing filler for rubber. This is because carbon black has a high reinforcing strength and abrasion resistance as compared with other fillers. In recent years, under social requirements for energy saving and resource saving, a reduction in heat generation of a compounded rubber has been required at the same time particularly for saving fuel consumption of automobiles.
Silica is known as a heat generation-reducing filler for a compounded rubber. Particles of silica tend to coagulate due to a hydrogen bond of a silanol group which is a surface functional group of silica, and therefore a kneading time has had to be extended in order to improve a dispersion of silica particles into rubber. In particular, since the surface of silica is hydrophilic due to characteristics of a silanol group thereof while rubber is lipophilic, there has been the defect that because of mutual repulsion, it is difficult to mix and disperse them and therefore sufficiently good dispersion cannot be secured.
Further, the surface of silica particles is acid, and therefore silica has the defect that since silica adsorbs a base substance used as a vulcanization accelerator when a rubber composition is vulcanized, the vulcanization does not go on sufficiently, which results in the poor elastic modulus.
In order to overcome these defects, a silica-blended rubber composition is blended with a silane coupling agent. Usually, a reaction mechanism of silane coupling agents is considered as follows.
In general, it is estimated that a silane coupling agent used in the rubber industry decreases, as shown in FIG. 2, silanol groups present on the surface of silica particles by dehydrate-condensation reaction (coupling reaction of silica with the silane coupling agent) of the silanol groups on the surface of silica with silanol groups produced by hydrolysis of alkoxyl groups contained in the silane coupling agent to thereby improve the dispersion of silica into the rubber and that the silane coupling agent is chemically combined with rubber molecules by a reaction (gelation) of another functional group of the silane coupling agent with the rubber molecules to form a reinforcing structure.
The preceding coupling reaction of silica with the silane coupling agent is hard to take place at low temperatures, and the reaction is considered to go on at temperatures of 140.degree. C. or higher in a rubber kneading process. In addition, there is the problem that when the coupling reaction described above proceeds insufficiently, egg-blisters are produced. Further, since the coupling reaction is an exothermic reaction, temperatures of 170.degree. C. or higher are reached in a short time.
On the other hand, high temperatures of 170.degree. C. or higher brings about a three-dimensional bridging reaction of rubber and increases rapidly the viscosity.
Accordingly, the existing state is that a sufficiently long reaction time of silica with a silane coupling agent cannot be taken in an actual rubber kneading step and that in order to carry out sufficiently the coupling reaction of silica with the silane coupling agent, a step of kneading--cooling--kneading--is repeated many times, usually 5 times or more. Accordingly, problems are present in terms of the workability and the productivity. Another existing state is that the dispersion of silica has not yet reached a sufficiently high level and in particular, the industrially good dispersion of silica has not been obtained.
Disclosed in Japanese Patent Application Laid-Open No. Hei 7-196850 is a rubber composition obtained by a mechanical treatment including at least one heat treatment of a copolymer having specific physical properties comprising conjugated diene and aromatic vinyl and silica having specific physical properties at temperatures falling in a range of 130 to 180.degree. C., preferably 145.degree. C. or higher.
In this rubber composition described in Japanese Patent Application Laid-Open No. Hei 7-196850, the copolymer having specific physical properties and silica having specific physical properties are combined and subjected to a heat treatment at temperatures falling in the preceding range of 130 to 180.degree. C., preferably 145.degree. C. or higher in order to obtain a rubber composition having an excellent abrasion resistance. It is described only in the test F of the paragraph No. 53 to use a silane coupling agent, and little description on the silane coupling agent can be found. In addition, it is not disclosed at all to enhance a dispersibility of silica into rubber by controlling reaction temperatures in a coupling reaction of conventional silica with a silane coupling agent.